Bimodal polymers produced using two or more different catalyst types—bimetallic catalysts—are of increasing interest, especially in producing polyethylene and other polyolefins. See, for example, U.S. Pat. No. 5,525,678. However, problems exist in using these bimetallic catalysts, especially in the gas phase. One problem is catalyst activity, which should be as high as possible in order to economize the process, as catalysts costs are significant.
One method of improving catalyst efficiency in gas phase processes is to improve upon the catalyst used in the process. A promising class of single-site catalysts for commercial use includes those wherein the metal center has at least one extractable fluorine (or fluorine “leaving group”). Disclosures of such catalysts include U.S. application Ser. No. 20020032287; WO 97/07141; DE 43 32 009 A1; EP-A2 0 200 351; EP-A1 0 705 849; E. F. Murphy, et al., Synthesis and spectroscopic characterization of a series of substituted cyclopentadienyl Group 4 fluorides; crystal structure of the acetylacetonato complex [(acac)2(η5-C5Me5)Zr(μ-F)SnMe3Cl], DALTON, 1983 (1996); A. Herzog, et al., Reactions of (η5-C5Me5)ZrF3, (η5-C5Me4Et)ZrF3, (η5-C5Me5)HfF3, and (η5-C5Me5)TaF4 with AlMe3, Structure of the First Hafnium-Aluminum-Carbon Cluster, 15 ORGANOMETALLICS 909-917 (1996); F. Garbassi, et al., JOURNAL OF MOLECULAR CATALYSIS A: CHEMICAL 101 199-209 (1995); and W. Kaminsky, et al., Fluorinated Half-Sandwich Complexes as Catalysts in Syndiospecific Styrene Polymerization, 30(25) MACROMOLECULES 7647-7650 (1997). Use of such single site catalyst components in a olefin polymerization system is desirable, especially in gas-phase polyethylene polymerization. It would be desirable to further improve upon this system, especially for bimodal gas phase polymerization processes. The present invention is directed towards solving this and other problems.